"When you have a sensor you want the thing you want to detect to come up to the surface, but it can take time," says senior author Professor Justin Gooding, of the UNSW School of Chemistry and the Australian Centre for Nanomedicine.

"For example if you are trying to find a microbe in water it may take three weeks to find it."

"So we don't make the analyte (item of interest) find the sensor, we make the sensor find the analtye. We break the sensor into very small pieces, send it out and then use a magnetic field to bring it back."

An added bonus of this approach is an increase in sensitivity, says Gooding, because the sensors pick up all the analyte in the fluid.

In a test of its sensitivity, published in the journal Angewandte Chemie, the biosensor was able to detect the antibiotic, enrofloxacin, at levels as low as one nanogram in one litre of milk within 40 minutes.

Divide and monitor

Gooding says the biochemiresistor can detect a single molecule, but he believes improvements on that level of sensitivity are achievable.

The biochemiresistor uses gold-coated magnetic particles about 150 nanometres in size that are coated with antibodies to the compound, or analyte, that needs to be detected.

The nanoparticles are dispersed through the liquid to be analysed and if the analyte is present some of the antibodies detach from the nanoparticles.

Using a magnet, the nanoparticles are then assembled back together into a film between two electrodes and the electrical resistance is measured.

The more analyte present in the sample, the more antibodies leave the nanoparticles and the lower the resistance in the film.

Gooding says the team is hoping to refine the technology to be able to detect rare cells in blood at a level of one cell in 10 million, to know whether dangerous cells are present.